An optical-fiber coupler is an important fiber optic element for communication systems and sensor systems which use light waves. In an optical fiber coupler, light is propagated through a fiber core and is radiated into a cladding or distribution-coupler so as to distribute at a predetermined degree of coupling to thereby enclose the light in the core again.
One way to make an optical-fiber coupler is to arrange two optical fibers in parallel with an unsheathed section of each fiber touching. A heat source, such as a gas flame, is applied to the center of the fibers until the fibers fuse together. While the fibers are still malleable, they are drawn out in the longitudinal direction to make a thin tapered portion of fiber. This tapered portion has an average outer diameter of about 30 .mu.m, which can be easily broken.
An apparatus is needed to reinforce and protect the thin tapered portion. Japanese Utility Model Unexamined Publication No. Hei-2-29008 and in Japanese Patent Unexamined Publication No. Sho-63-271208 describe reinforcing apparatuses.
An example of a conventional reinforcing apparatus is shown in FIG. 11 (prior art). After the fibers have been drawn out, an adhesive (2) is applied on both sides of the thin tapered portion. Then the optical-fiber coupler is sandwiched between two flat plates (3) to form a conventional reinforcing apparatus. This simple design is a weak structure because the plates do not form a solid case. Therefore, the plates can vibrate and allow the fibers to twist. Also, the thin tapered portion is exposed to the atmosphere where dust, dirt, smoke, etc., can attach to the optical-fiber coupler to cause leakage of light.
Another reinforcing apparatus which is slightly better is shown in FIG. 12 (prior art). A glass tube (4) is used to reinforce the optical-fiber coupler by applying the adhesive (2) to the optical fibers (1) and securing the coupler to the glass. This is a stronger apparatus than the design shown in FIG. 11 (prior art), but it is difficult to apply the adhesive to the optical coupler while it is in the glass tube (4). this structure also allows the thin tapered portion to be in contact with the atmosphere.
One solution is to make an apparatus shown in FIG. 13 (prior art). The optical-fiber coupler is housed in groove portion of the U-shaped base (5). An adhesive (2) is applied on each side of the thin tapered portion of the optical-fiber coupler and the cover (6) is placed on over the groove portion of the U-shaped base (5). The adhesive (2) holds the cover in place. The advantage of this reinforcing apparatus is that the thin tapered portion of the coupler is completely sealed within the reinforcing apparatus keeping all atmospheric dust away from the exposed fused fibers. The disadvantage is that assembling the apparatus can be troublesome.